The objects made by the additive manufacturing technique referred to as three-dimensional printing, or 3D printing, are also referred to as parts. The parts are typically made by spreading successive layers of powder, here specifically metal powder, in a powder bed, and by releasing a binder into each powder layer. The binder binds regions within each layer to each other and to regions in the layer beneath it. Once all the necessary layers have been spread and bound, one or more three-dimensional parts have been produced. These parts are referred to as “green” parts. They still contain the binder. The parts need to be cleaned so as to remove the excess, non-bound powder, before they can be de-binded, for instance by controlled heating, and post-processed, for instance by sintering or other types of curing.
The green parts are relatively fragile. Moreover, they currently have a dimensional accuracy of several tens of micrometers. For a discussion of dimensional accuracy see “An Investigation of Dimensional Accuracy of Parts Produced by Three-Dimensional Printing” by M. N. Islam et al. in Proceedings of the World Congress on Engineering 2013 Vol. I, WCE 2013, Jul. 3-5, 2013, London, U. K. The dimensional accuracy is expected to improve. The surface roughness of the green parts is in the range from several to several tens of micrometers. It is largely determined by the grain size and the physical properties of the metal powder and by the printer resolution. Efforts are made to lower the surface roughness and consequently improve the surface quality of the post-processed parts. It should be evident that any accidental removal of bound powder from the green parts will significantly influence the dimensional accuracy and the surface quality of the parts. This is to be avoided. The invention addresses this need and provides a system for non-destructively cleaning such green parts.
When designing such a system, however, the health and safety of the operator using the cleaning system is also to be considered because powders, here specifically metal powders, pose health and safety hazards. The main hazards have been split into two, explosion within a process and personal health hazards. For both types of hazards see Chapter 15 of “Fundamentals of Particle Technology” by Richard G. Holdich, Midland Information Technology and Publishing, Shepshed, Leicestershire, U. K, 2002. A short summary is as follows. Very fine metal powders as used in 3D printing become airborne. Many of these, e.g. magnesium and aluminum, have been found to be explosive provided that certain conditions are met, amongst them a critical dust concentration, a certain temperature and an ignition source of sufficient energy. The present invention seeks to avoid meeting these conditions. Powders having an average particle diameter of 10 μm or less can also be absorbed into the body and give rise to chemical or biochemical reactions that are potentially dangerous for the skin, the eyes and the lungs. The present invention seeks to reduce the exposure of the operator to the metal powder removed in the cleaning process.
The invention thus proposes a flow cabinet system for cleaning objects such as green parts made by a three-dimensional printing process. Flow cabinets are generally known. However, the inventors found that none of the known systems addressed the specific needs described above.
For instance, Biosafety in Microbiological and Biomedical Laboratories (BMBL), 5th Edition (December 2009) discloses in Appendix A various types of biological safety cabinets designed to provide personal, environmental and product protection. A horizontal laminar flow cabinet and a vertical laminar flow cabinet are disclosed in Appendix A; however, whilst these cabinets may sufficiently protect the product, they protect the operator only insufficiently.
U.S. Pat. Nos. 3,599,375, and 4,300,318 show examples of a cabinet system comprising a cabinet and a handheld gun. By means of the gun an operator is able to direct abrasive material to an object to be abraded inside the cabinet. If abrasion can be considered “cleaning” at all, it has to be considered a destructive cleaning since the abrasion removes part of the material to be cleaned. The cabinet system of U.S. Pat. No. 4,300,318, for instance, generates relatively large amounts of powder, dust, debris, abrasive material and particles dislodged from the object, such that the cabinet system uses a vacuum motor and additionally requires a rack and a hopper connected to the vacuum motor to remove these amounts from the cabinet.
Therefore, it is the goal of the present invention to provide a flow cabinet system optimized to clean objects in an efficient and operator protective manner, in particular objects manufactured with additive manufacturing.